1. Field of the Invention
The present invention relates to a dielectric ceramic composition having a resistance to reduction and to a multi-layer ceramic capacitor or other electronic device using the same, more particularly relates to a dielectric ceramic composition having a capacity-temperature characteristic satisfying the X8R characteristic of the Electronic Industries Association (EIA) standard (xe2x88x9255 to 150xc2x0 C., xcex94C=xc2x115% or less) and capable of improving mechanical strength of an electronic device.
2. Description of the Related Art
A multi-layer ceramic capacitor, one type of electronic device, is being broadly used as a compact, large capacity, high reliability electronic device. The number used in each piece of electronic equipment has also become larger. In recent years, along with the increasing miniaturization and improved performance of equipment, there have been increasingly stronger demands for further reductions in size, increases in capacity, reductions in price, and improvements in reliability in multi-layer ceramic capacitors.
Multi-layer ceramic capacitors are normally produced by stacking a paste for forming the internal electrode layers and a paste for forming the dielectric layers using the sheet method or printing method etc. and then cofiring the internal electrode layers and dielectric layers in the stack together.
As the electroconductive material for the internal electrode layers, generally Pd or a Pd alloy is used, but since Pd is high in price, relatively inexpensive Ni, Ni alloys, and other base metals have come into use. When using a base metal as the electroconductive material of the internal electrode layers, firing in the atmosphere ends up oxidizing the internal electrode layers and therefore the cofiring of the dielectric layers and internal electrode layers has to be done in a reducing atmosphere. When being fired in a reducing atmosphere, however, the dielectric layers end up being reduced and becoming lower in specific resistance. Therefore, nonreducing type dielectric materials are being developed.
Further, a capacitor is also required to be excellent in temperature characteristics. In particular, in some applications, it is desired that the temperature characteristics be smooth under harsh conditions. In recent years, multi-layer ceramic capacitors have come into use for various types of electronic equipments such as the engine electronic control units (ECU) mounted in engine compartments of automobiles, crank angle sensors, antilock brake system (ABS) modules, etc. These electronic equipments are used for stabilizing engine control, drive control, and brake control, and therefore are required to have excellent circuit temperature stability.
The environment in which these electronic equipments are used is envisioned to be one in which the temperature falls to as low as xe2x88x9220xc2x0 C. or so in the winter in cold areas or the temperature rises to as high as +130xc2x0 C. or so in the summer right after engine startup. Recently, there has been a trend toward reduction of the number of wire harnesses used for connecting electronic apparatuses and the equipment they control. Electronic apparatuses are also being mounted outside of the vehicles in some cases. Therefore, the environment is becoming increasingly severe for electronic apparatuses. Accordingly, capacitors used for these electronic apparatuses have to have smooth temperature characteristics in a broad temperature range.
As temperature-compensating capacitor materials superior in temperature characteristics, (Sr, Ca)(Ti, Zr)O3 based, Ca(Ti, Zr)O3 based, Nd2O3-2TiO2 based, La2O3-2TiO2 based, and other materials are generally known, but these compositions have extremely low specific dielectric constants (generally less than 100), so it is substantially impossible to produce a capacitor having a large capacity.
As a dielectric ceramic composition having a high dielectric constant and a smooth capacity-temperature characteristic, a composition comprised of BaTiO3 as a main component plus Nb2O5xe2x80x94Co3O4, MgOxe2x80x94Y, rare earth elements (Dy, Ho, etc.), Bi2O3xe2x80x94TiO2, etc. is known. Looking at the temperature characteristic of a dielectric ceramic composition comprising BaTiO3 as a main component, since the Curie temperature of pure BaTiO3 is close to about 130xc2x0 C., it is extremely difficult to satisfy the R characteristic of the capacity-temperature characteristic (xcex94C=xc2x115% or less) in the region higher in temperature than that. Therefore, a BaTiO3 based high dielectric constant material can only satisfy the X7R characteristic of the EIA standard (xe2x88x9255 to 125xc2x0 C., xcex94C=xc2x115% or less). If only satisfying the X7R characteristic, the material is not good enough for an electronic apparatus of an automobile which is used in the above-mentioned harsh environments. The above electronic apparatus requires a dielectric ceramic composition satisfying the X8R characteristic of the EIA standard (xe2x88x9255 to 150xc2x0 C., xcex94C=xc2x115% or less).
To satisfy the X8R characteristic in a dielectric ceramic composition comprised of BaTiO3 as a main component, it has been proposed to shift the Curie temperature to the high temperature side by replacing the Ba in the BaTiO3 with Bi, Pb, etc. (Japanese Unexamined Patent Publication (Kokai) No. 10-25157 and No. 9-40465). Further, it has also been proposed to satisfy the X8R characteristic by selecting a BaTiO3+CaZrO3+ZnO+Nb2O5 based composition (Japanese Unexamined Patent Publication (Kokai) No. 4-295048, No. 4-292458, No. 4-292459, No. 5-109319, and No. 6-243721). In each of these compositions as well, however, since Pb, Bi, and Zn which are easily vaporised and scattered are used, firing in air or another oxidizing atmosphere becomes a prerequisite. Therefore, there are the problems that it is not possible to use an inexpensive base metal such as Ni for the internal electrodes of the capacitor and it is necessary to use Pd, Au, Ag, or another high priced precious metal.
Furthermore, in a dielectric ceramic composition of the related art, there are problems of not having resistance to repeated heat impulses from a low temperature to a high temperature, etc. so improvement in mechanical strength is required.
On the other hand, a method of improving the strength by forming acicula crystalline on outer surface regions of the both ends in the stack direction of a ceramic layered body has been proposed (Japanese Unexamined Patent Publication (kokai) No. 9-312234). In the method described in the publication, an oxide paste composed of Tio2 as a main component is applied on the surface of the ceramic layered body, which is dried and then subjected to heat processing, consequently, the acicula crystalline is formed on the outer surface region of the ceramic. It is considered that the acicula crystalline is deposited substance of Ba4Ti13O30, Ba6Ti17O40, etc.
In the method described in the above publication, however, since the ceramic base is reduced when firing the ceramic layered body in the reducing atmosphere, acicula crystalline cannot be formed inside or close to the center of the ceramic layered body. Accordingly, forming of the acicula crystalline is limited only near the outer surface of the ceramic layered body and the mechanical strength is not sufficiently improved. Thus, when a thickness of a capacitor cover for protecting the ceramic layered body cannot be sufficiently secured, an electronic device having sufficient strength able to be used in practice cannot be obtained.
An object of the present invention is to provide a dielectric ceramic composition having a high specific dielectric constant and a capacity-temperature characteristic satisfying the X8R characteristic of the EIA standard (xe2x88x9255 to 150xc2x0 C., xcex94C=xc2x115% or less), and able to be fired in a reducing atmosphere, and further, to provide a multi-layer ceramic capacitor or other electronic device using this dielectric ceramic composition.
To attain the above object, a dielectric ceramic composition according to a first aspect of the present invention comprises:
a main component of BaTiO3,
a first subcomponent including at least one compound selected from MgO, CaO, BaO, SrO and Cr2O3,
a second subcomponent of (Ba, Ca)xSiO2+x (where, x=0.8 to 1.2),
a third subcomponent including at least one compound selected from V2O5, MoO3, and WO3,
a fourth subcomponent including an oxide of R1 (where R1 is at least one element selected from Sc, Er, Tm, Yb, and Lu), and
a fifth subcomponent including an oxide of R2 (where R2 is at least one element selected from Y, Dy, Ho, Tb, Gb and Eu), wherein
the ratios of the subcomponents to 100 moles of the main component of BaTiO3 are:
first subcomponent: 0.1 to 3 moles,
second subcomponent: 2 to 10 moles,
third subcomponent: 0.01 to 0.5 mole,
fourth subcomponent: 0.5 to 7 moles (where the number of moles of the fourth subcomponent is the ratio of R1 alone), and
fifth subcomponent: 2 to 9 moles (where the number of moles of the fifth subcomponent is the ratio of R2 alone).
Note that the ratio of Ba and Ca in the second subcomponent may be any, and only one of the two may be contained, as well.
Preferably, the dielectric ceramic composition according to the present invention further comprises as a sixth subcomponent MnO, the content of the sixth subcomponent being not more than 0.5 mole with respect to 100 moles of the main component of BaTiO3.
Preferably, the total content of the fourth subcomponent and the fifth subcomponent are not more than 13 moles with respect to 100 moles of the main component of BaTiO3 (where the numbers of moles of the fourth subcomponent and fifth subcomponent are the ratios of R1 and R2 alone).
A dielectric ceramic composition according to a second aspect of the present invention is comprised of a dielectric base phase containing BaTiO3 as a main component, and plate-shaped or acicula deposition phase existing in the dielectric base phase. In the dielectric ceramic composition according to the second aspect of the present invention, materials of the deposition phase is not specifically limited, but composed of for example a composite oxide of Y, Yb and Ti, etc.
An electronic device according to the present invention is not specifically limited as far as it is an electronic device having a dielectric layer, which is for example a multi-layer ceramic capacitor device having a capacitor device body comprised of dielectric layers and internal electrode layers alternately stacked. In the present invention, the dielectric layer is comprised of any one of the above dielectric compositions. As the electroconductive material included in the internal electrode layers is not specifically limited, but for example Ni or an Ni alloy.
According to the present invention, a dielectric ceramic composition satisfying a X8R temperature characteristic (xe2x88x9255 to 150xc2x0 C., xcex94C=xc2x115% or less), superior in a resistance to reduction and mechanical strength can be obtained.
In the present invention, the mechanical strength is improved by the ratio of R2 being 2 moles or more with respect to 100 moles of BaTiO3 and by adding the same together with R1. Such a new knowledge was found for the first time by the present inventors.
Note that when the ratio of R1 is less than 0.5 moles, it tends to become difficult to obtain the X8R characteristic, while when R2 exceeding 9 moles or R1 exceeding 7 moles is added to the composition, there is a tendency that sinterability is deteriorated and preferable electric characteristics cannot be obtained, and also, sufficient mechanical strength cannot be obtained.
Also, it is observed in the present invention that mechanical strength is improved in the dielectric ceramic composition wherein plate-shaped or acicula deposition phase is detected inside the dielectric base phase comparing with the one wherein the deposition phase is not detected.
Also, the dielectric ceramic composition according to the present invention is capable of being fired in a reducing atmosphere, and can be used for multi-layer chip capacitor materials having internal electrodes composed of base metals such as Ni and Ni alloy, etc.